More particularly but not exclusively, said lifting surface at the trailing edge of which the flap is mounted, is a helicopter rotor blade, in particular a blade of the helicopter main advance and lifting rotor. However, it may also consist of a rotorcraft blade or that of a convertible aircraft or the lifting surface of such an aircraft such as a horizontal stabilizer wing, for instance.
It is known that the flap trailing edge of a helicopter rotor blade is controlled for its angle of attack being steered through actuating means that:                either include one customary actuator, for instance of the electromagnetic type;        or consist of “intelligent” materials of the piezo-electric, magneto-restrictive or shape memory type.        
In practice, it is found that on a blade of this type referred to as “active”, excessive steering limitations on the flap intervene systematically thus reducing the performances of the blade. In particular, these limitations are due to the high hinge moment of the flap, i.e. the moment that has to be overcome to maneuver the flap about its axis of rotation. Therefore, for example, means of actuation based on piezo-electric material have energy and mass characteristics that are insufficient to overcome the entire hinge moment and control the flap at the frequencies generally required during a flight, efficiently control the vibration, or significantly decrease the aircraft sound level. Similarly, to overcome the hinge moment, the customary operating resources call for relatively heavy and bulky plants that are often difficult to accommodate in the blade.
Consequently, the known actuating resources will not permit optimum steering (in particular in terms of performance) of the flap because more particularly of the existence of too high a hinge moment.